Variable stiffness actuator with electrically modulated stiffness

ABSTRACT

A dielectric elastomer system (DES) variable stiffness actuator (VSA) is provided. In an embodiment, the DES VSA includes a variable stiffness module (VSM). The VSM includes a DES that softens when energized and stiffens when unpowered, an outer frame, and an inner frame member. The stiffness of the DES is variable. The outer frame supports the DES and the inner frame member, which is disposed within the DES. The inner frame member is configured to be displaceable with respect to the outer frame. The DES VSA also includes an actuation motor mechanically coupled to the inner frame member that is configured to cause a force to be applied to the inner frame member and the actuation motor is configured to control an equilibrium position of the DES VSA.

PRIORITY

This application claims priority to U.S. Provisional Application No.62/549,319, filed Aug. 23, 2017, which is incorporated herein byreference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under grant numberHD080349 awarded by the National Institutes of Health (NIH). Thegovernment has certain rights in the invention.

BACKGROUND 1. Field

The disclosure relates generally to robotics and actuators, and morespecifically, to compliant actuators and variable stiffness actuators.

2. Description of the Related Art

Variable stiffness actuators (VSAs) are electromechanical actuatorsused, for example, for legged and gait assistance robots. VSAs areinherently compliant actuators with variable stiffness. VSAs haveseveral advantages as compared to rigid electromechanical actuators forhuman-interactive and biomimetic robots. A VSA can use its compliantelement to store and return mechanical energy, which may be moreefficient than using a motor and battery to convert mechanical energy toelectrical energy and then to chemical energy, and vice versa. Theability to store and return energy can improve the efficiency of robotsperforming cyclic tasks such as legged locomotion. If placed between theVSA's motor and load (as is usually the case), the compliant elementdecouples the motor and load inertias, which reduces the dissipation ofenergy caused by inelastic collisions. By compliantly absorbing impacts,the compliant element acts as a hardware safety feature that can preventharm to humans or the actuator itself. In contrast, rigid actuators canonly respond compliantly to impacts within their control system'sbandwidth. A VSA can tune its compliance in order to minimize energyconsumption or peak power requirements. The optimal choice of stiffnessfor these goals depends on the task and load, so VSAs are especiallyvaluable in human-interaction applications where these conditions canvary greatly.

Despite these advantages, VSAs are difficult to implement in robotsbecause of the additional components their variable stiffness mechanismsadd to the actuator. These components usually include moving parts andan additional motor, and they increase the VSA's weight, volume, andcost, and decrease its durability compared to rigid actuators andfixed-stiffness series elastic actuators. In order to reduce weight,volume, and cost, the stiffness modulation motor is often much lesspowerful than the main drive motor, which limits the VSA's stiffnesschanging speed and may make it impossible for the VSA to changestiffness under load, though some VSAs can change stiffness in less than0.5 seconds (s). Antagonistic VSA designs typically can change stiffnessrapidly and under load, but they require more energy to change stiffnessthan other VSA designs.

Therefore, it would be desirable to have a method and apparatus thattake into account at least some of the issues discussed above, as wellas other possible issues. For example, it would be desirable to have amethod and apparatus that overcome a technical problem with theexcessive weight, the excess energy consumption, and the durabilityissues.

SUMMARY

According to one embodiment of the present invention, a dielectricelastomer system (DES) variable stiffness actuator (VSA) is provided.The DES VSA includes a variable stiffness module (VSM). The VSM includesa DES that softens when energized and stiffens when unpowered, an outerframe, and an inner frame member. The outer frame supports the DES andthe inner frame member, which is disposed within the DES. The innerframe member is configured to be displaceable with respect to the outerframe. The DES VSA also includes an actuation motor mechanically coupledto the inner frame member that is configured to cause a force to beapplied to the inner frame member, and the actuation motor is configuredto control an equilibrium position of the DES VSA.

According to another embodiment of the present invention, a dielectricelastomer system (DES) variable stiffness actuator (VSA) is provided.The DES VSA includes a compliant membrane. The stiffness of thecompliant membrane is adjustable by electrically energizing thecompliant membrane. The compliant membrane is stiffer when unpoweredthan when powered. The DES VSA also includes a stiffness controllerconnected to the compliant membrane and configured to control thestiffness of the compliant membrane. The DES VSA also includes anactuation motor connected to the compliant membrane and configured tocontrol an equilibrium position of the DES VSA. The DES VSA alsoincludes a connector connecting the compliant membrane and the actuationmotor to a load.

According to another embodiment of the present invention, a dielectricelastomer system (DES) variable stiffness actuator (VSA) is provided.The DES VSA includes a plurality of elastomer sheets. Each of theelastomer sheets is configured to soften when energized with an electricfield and to become stiffer when unenergized. The DES VSA also includesa stiffness controller connected to the plurality of elastomer sheets.The stiffness controller is configured to control the stiffness of theplurality of elastomer sheets via application of a control voltage tothe plurality of elastomer sheets. The DES VSA also includes a ballscrew and an actuation motor coupled to the ball screw. The actuationmotor is configured to adjust an equilibrium position of the DES VSA.The DES VSA also includes an input block configured to mechanicallycouple first ends of the elastomer sheets to the ball screw. The DES VSAalso includes an output connection point; and a plurality of outputblocks each configured to couple one or more of respective one or moreof the plurality of elastomer sheets to the output connection point. Theoutput connection point is configured to receive a load.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the illustrativeembodiments are set forth in the appended claims. The illustrativeembodiments, however, as well as a preferred mode of use, furtherobjectives and features thereof, will best be understood by reference tothe following detailed description of an illustrative embodiment of thepresent disclosure when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a schematic diagram of a typical robotic actuator;

FIG. 2 is a schematic diagram of a DES VSA in accordance with anillustrative embodiment;

FIG. 3 is a schematic diagram of a DES VSA in accordance with anillustrative embodiment;

FIG. 4 is a perspective view of a DES VSA in accordance with anillustrative embodiment;

FIG. 5 is a perspective view of a DES VSA in accordance with anillustrative embodiment;

FIG. 6A is a diagram of a DES module depicted in accordance with anillustrative embodiment;

FIG. 6B is a diagram of an alternate DES module depicted in accordancewith an illustrative embodiment;

FIG. 7 is a cross-section view of a DES VSA in accordance with anillustrative embodiment;

FIG. 8A is a cross-section view of a DES module in accordance with anillustrative embodiment;

FIG. 8B is an expanded view of a portion of the DES module depicted inFIG. 8A;

FIG. 9 is a plot of a VSA signature showing independent modulation ofstiffness and equilibrium position, according to test results obtainedfrom an illustrative embodiment of the DES VSA;

FIGS. 10A, 10B, and 10C show plots of tensile test results showing theDES VSA's stiffness change;

FIG. 11 shows a plot of tensile test results for multiple voltages; and

FIG. 12 is a block diagram of a data processing system in accordancewith an illustrative embodiment.

DETAILED DESCRIPTION

The illustrative embodiments relate to VSAs. Dielectric elastomersystems (DESs) soften when charged with a constant voltage, providingelectric modulation of stiffness. Disclosed herein are illustrativeembodiments of VSAs that include a DES that softens when energized andstiffens when unpowered. In an embodiment, DESs are used to create a DESVSA with a mechanically simpler variable stiffness mechanism. In anembodiment, the DES VSA provides independent control of stiffness andequilibrium position with force output of, for example, 140 Newtons (N).No prior DES VSA can match this combination. In an embodiment, thedisclosed variable stiffness mechanism has no sliding or rolling parts,and no bushings or bearings, yet still provides low-power stiffnessmodulation. In an embodiment, the DES VSA softens up to 52% and requiresmerely 262 milliwatts (mW) to hold that softened state. Unlike priorVSAs, embodiments of the disclosed DES VSA default to stiff behaviorwhen unpowered. This results, for example, in reduced energy consumptionfor legged and gait-assistance robots during long periods of standing.

Perhaps the most promising electroactive polymers for “human-scale”robotics are dielectric elastomer systems (DESs) because they offer aunique combination of high stress and strain capacities, work density,strain rate, and energy conversion efficiency. DESs are simplemechanical devices, consisting of only a few components, none of whichroll or slide. The core DES component is a thin layer of dielectricelastomer coated on its top and bottom faces with stretchableelectrodes. When a constant voltage is applied across the electrodes,the stiffness of the DES in the plane of the dielectric layer decreases.When no voltage is applied, the DES defaults to a stiff state.

Disclosed herein are illustrative embodiments that use this softeningproperty of DESs to create a variable stiffness actuator (VSA) with asimplified variable stiffness mechanism. As used herein, VSA refers toas an actuator that can change the stiffness of its compliant element.Such a device can modulate its stiffness to compensate for changes ingait parameters and loading conditions.

No prior DES VSA can control its inherent stiffness and equilibriumposition independently and simultaneously, and few can exert forcesgreater than 100N. Because a VSA's actuation motor is compliantlycoupled to the actuator output, its motion does not set the VSA's outputposition directly but rather its equilibrium position, the outputposition when no load is applied to the VSA. DES diaphragm modules,developed for variable stiffness suspensions, can vary their stiffnessbut not their equilibrium position, and early works did not demonstratea force output greater than 1N. Coupling one or more diaphragm moduleswith a biasing mechanism results in a dielectric elastomer actuator thathas one degree of freedom. Such an actuator changes both its stiffnessand equilibrium position, but these two changes are coupled. Using asecond DES diaphragm module as the biasing mechanism adds a seconddegree of freedom, but only partially decouples the control of stiffnessand equilibrium position. A prior art DES orthosis can vary stiffnessand equilibrium independently, but does so by using closed-loop controlrather than modulating its inherent stiffness.

Embodiments of the disclosed DES VSA are simpler mechanically, yet stillmatches the functionality of the VSA state of the art. The variablestiffness mechanism of an embodiment of the disclosed DES VSA ismechanically simple, having no rolling or sliding parts, and can changestiffness while loaded. Unlike state-of-the-art VSAs, the disclosed DESVSA softens when energized and is thus “default-stiff,” which isbeneficial for applications requiring rigid, unpowered behavior such asrobotic prosthetic legs. In an embodiment, the disclosed DES VSA canexert 140N making it more suitable for larger-scale robotic applicationsthan previous DES VSAs. Embodiments of the disclosed DES VSA can alsocontrol their stiffness and equilibrium position independently andsimultaneously unlike previous DES VSAs. In an illustrative embodiment,the disclosed DES VSA leverages the advantages of an electric motor anda DES: an easily-controlled electric motor with a ball screwtransmission sets the equilibrium position, and inherently-compliant DESmodules create and modulate actuator compliance.

With reference now to the Figures and, in particular, with reference toFIG. 1, a schematic diagram of a typical robotic actuator 100 isdepicted. The actuator 100 includes an electric motor 102 and atransmission 104 that couples to a load 106.

FIG. 2 is a schematic diagram of a DES VSA 200 in accordance with anillustrative embodiment. DES VSA 200 includes an electric motor 202, atransmission 204, and a compliant element 206 that couples to a load208. The compliant element 206 may be treated mathematically as a springwith a spring constant, k. However, the spring constant, k, is notnecessarily constant, but may vary with the strain, strain rate, andtemperature of the compliant element 206. In an embodiment, the electricmotor 202 is coupled to an electric motor controller (not shown) thatcontrols the operation of the electric motor 202. The electric motorcontroller may include or be implemented as a data processing system. Inan embodiment, the compliant element 206 has variable stiffnesscontrolled by applying varying levels of electric voltage to thecompliant element 206. Thus, in an embodiment, the spring constant, k,varies, depending on the level of the electric voltage applied. Theaddition of the compliant element 206 results in numerous benefits overa rigid actuator as depicted in FIG. 1. The compliant element 206 storesmechanical energy from the load 208 and returns mechanical energy to theload 208 thereby improving the efficiency of, for example, robotsperforming cyclic tasks such as legged locomotion. Additionally, thecompliant element 206 decouples the motor 202 and load 208 inertias,which reduces the dissipation of energy caused by inelastic collisions.Furthermore, by compliantly absorbing impacts, the compliant element 206acts as a hardware safety feature that can prevent harm to humans or tothe actuator itself.

In an embodiment, compliant element 206 includes a variable stiffnessflexible membrane. The stiffness of the variable stiffness flexiblemembrane is adjustable by electrically energizing the variable stiffnessflexible membrane. The variable stiffness flexible membrane is stifferwhen unpowered. In an embodiment, a stiffness controller (not shown) isconnected to the compliant element 206 and is configured to control thestiffness of the variable stiffness flexible membrane. The stiffnesscontroller includes an electric power supply that, in an embodiment,provides a voltage to compliant electrodes that are disposed on eitherside of the variable stiffness flexible membrane. The stiffnesscontroller may also include or be implemented as a data processingsystem for controlling the voltage applied to the compliant element 206.The electric motor controller and the stiffness controller may beincluded in the same data processing system. Varying the voltage acrossthe variable stiffness flexible membrane causes the stiffness of thevariable stiffness flexible membrane to change.

In one embodiment, the variable stiffness flexible membrane is orientedsuch that the plane of the membrane is parallel to a direction of forceapplied by the actuation motor to the load. FIGS. 3 and 4 and theirdescription below provide more details regarding an illustrative exampleof this embodiment.

In another embodiment, the variable stiffness flexible membrane isoriented such that the plane of the membrane is perpendicular to adirection of force applied by the motor 202 to the load. Illustrativeexamples of this embodiment are shown in FIGS. 5, 6A, 6B, 7, 8A, and 8Band described below.

The motor 202 is configured to control an equilibrium length of theactuator 200.

FIG. 3 is a schematic diagram of a DES VSA 300 in accordance with anillustrative embodiment. DES VSA 300 is an example of a DES VSA 200depicted in FIG. 2. DES VSA 300 includes an electric motor 302, a ballscrew 304, an input block 306, an output block 308 and two sets ofdielectric elastomer systems (DESs) 310 designated as set A and set B inFIG. 3. The DESs 310 are arranged in planes on either side of the ballscrew. A first end of the DESs 310 of set A are connected to one end ofthe output block 308 and a second end of the DESs 310 of set A areconnected to the input block 306. A first end of the DESs 310 of set Bare connected to output block 308 at the end opposite from the end ofthe output block 308 to which the DESs 310 of set A are connected. Asecond end of the DESs 310 of set B are connected to the input block306. The plane of the DESs 310 are planar sheets and the plane of eachof the DESs 310 lie in a plane perpendicular to the z-axis as z-axis isdefined in FIG. 3. In an embodiment, the input block 306 is an inputclamp and the output block 308 is an output clamp.

In an embodiment, the DESs 310 include alternating layers of dielectricelastomer sheets 312 and positive compliant electrodes 314 and negativecompliant electrodes 316. The compliant electrodes 314, 316 allow theflexibility of the dielectric elastomers 312 to be controlled andadjusted as needed.

The electric motor 302 drives the ball screw 304 causing the input block306 to translate in the x-direction. The input block 306 is compliantlyconnected to the output block 308 through two sets of DESs 310 (depictedas springs). The load (F) is applied to the output block 308.

Variable stiffness mechanisms using conventional mechanical componentstend to be complex and bulky. A dielectric elastomer system (DES) 310 isan alternative variable stiffness device that is mechanically simple andlightweight. A DES 310 consists of an elastomer 312, a rubbery polymer,sandwiched between compliant electrodes 314, 316 as shown in FIG. 3.When the electrodes 314, 316 are electrically charged (oppositely), theyattract each other and cause a compressive force on the sandwichedelastomer 312 in the z-direction. Because of this compression, theelastomer 312 expands in the x- and y-directions. This expansive effectcan be harnessed for actuation.

Charges on the electrodes 314, 316 also cause the stiffness of the DES310 to change.

An equation that relates the change of stiffness to the applied voltagefor a planar DES with one end fixed, a load applied to its opposite end,and its width constrained is:k _(eff) =k ₀ −bV ²  (1)Eq. 1 says that the effective DESstiffness in the actuation direction,k_(eff), is reduced from the stiffness of the uncharged device, k₀, bythe square of the applied voltage, V, scaled by a constant, which isdependent on the elastomer's dimensions and electrical permittivity. Inan embodiment, a DES VSA 300 with a DES 310 as its compliant elementtakes advantage of this effect so that the stiffness of the DES VSA 300is modulated electrically.

A direct drive ball screw 304 provides “gear reduction” in a compact,lightweight package and converts the rotary motion of the motor 302 tolinear motion that interfaces readily with the planar, dielectricelastomer sheets 312. Because of these benefits, in an embodiment, aball screw was selected to connect the motor 302 to the dielectricelastomer sheets 312.

There are many possible mechanical configurations for the DES 310.Stack, bending beam, diaphragm, and tube are some of the common options.The selection of a particular configuration has a direct impact on themaximum strain, required voltage, force output, power density, andfabrication complexity of the device. In an embodiment, the DES VSA 300uses rectangular DESs 310 stacked on each other. In this design, planar,dielectric elastomer sheets 312 are stacked in layers with electrodes314, 316 between the elastomers 312 as shown in FIG. 3. The stacks areclamped at their ends that are parallel to the y-axis, and then themotion of the input and output blocks 306, 308 stretches and relaxes thesheets in the x-direction. Some benefits of this configuration arecomparatively simple manufacturing, a simple mathematical model, andease of attachment to other mechanical components.

The DES VSA 300 design uses four stacks of DESs 310 configured to act asparallel springs that are intended to be solely in tension duringoperation. These stacks are depicted schematically as springs in FIG. 3.The DESs 310 on the motor 302 end of the DES VSA 300 (set A) pull on theoutput block 308 in opposition, antagonistically, to the force from theDESs 310 on the output end of the DES VSA 300 (set B). When set Aextends, set B contracts, and vice-versa.

One benefit of this configuration is that expansion of the DES 310 inthe x-y plane does not alter the equilibrium position of the outputblock 308. A second benefit is that because the antagonisticconfiguration is designed to keep the sheets 312 in tension, the DES VSA300 can use their elastic effects whether it receives a tension orcompression load.

In an embodiment, the elastomer material for the elastomer sheets 312 isa urethane polymer with, for example, characteristics such as describedbelow in Table I. Its high relative permittivity (14) and highdielectric breakdown strength (60V/μm) enable it to undergo largestiffness reductions. Its failure strain and elastic modulus are wellsuited for the DES VSA 300.

In an embodiment, the dimensions of the sheets used in the stack DES 310configuration were constrained by several factors. First, the upperstiffness target constrained the overall geometry according to:

$\begin{matrix}{{k = {\frac{nzw}{l}Y}},} & (2)\end{matrix}$where k is the target stiffness value, n is the total number of sheets312, z is the thickness of each sheet 312, w is the width of each sheet312, and Y is the elastic modulus of the sheets 312. Second, in anembodiment, to minimize sheet manufacturing and actuator assembly time,the number of sheets 312, n, is minimized (sheet manufacturing can takedays depending on the process). In an embodiment, the manufacturingprocess limited the maximum length and width of the sheets 312 to beeither 10.2 cm or 9.2 cm. In order to minimize the actuation strain ofthe sheets 312, which is inversely proportional to their unstretchedlength, the sheet 312 length was set to the larger of these values (10.2cm). The width was then chosen to be 9.2 cm in order to minimize naccording to Eq. 2. Eq. 2 also suggests that z be maximized in order toreduce n, but increasing z also increases the voltage necessary toobtain the reduction between the upper and lower stiffness targets.Table I below summarizes the values selected for an embodiment of theDES VSA 300.

TABLE I Dielectric Elastomer System Parameters Parameter Value UnitsYoung's Modulus 2.5 MPa Max. Strain 260 % Relative permittivity 14 —Dielectric strength 60 V/μm Dimensions l × w × z (nominal) IndividualSheet 114 × 92.2 × 0.2 mm Active Area 63.5 × 92.2 × 0.2 mm

FIG. 4 is a perspective view of a DES VSA 400 in accordance with anillustrative embodiment. DES VSA 400 may be implemented as DES VSA 300in FIG. 3. DES VSA 400 includes a motor 402, a motor mount 403, a ballscrew 404, an input block 406, an output block 408, an output connectionpoint 409, DESs 410, and guide rods 411. Motor 402 may be implemented asmotor 302, ball screw 404 may be implemented as ball screw 304, inputblock 406 may be implemented as input block 306, output block 408 may beimplemented as output block 308, and DESs 410 may be implemented as DES310 depicted in FIG. 3 and described above.

The overall layout of the DES VSA 400 provides a compact deviceutilizing 1) a direct drive ball screw 404, 2) DESs 410, and 3) anantagonistic spring configuration. In an embodiment, the DES VSA 400 hasa weight of 734 grams (g) and an overall length of 33 centimeters (cm).The mechanical arrangement is as follows. The motor mount 403 is fixedin place by a pin joint on the underside of the DES VSA 400. The motormount 403 supports two guide rods 411 that serve as the backbone of theDES VSA 400. In an embodiment, screws hold the motor 402, implemented inan embodiment as a 70 W brushless DC motor (Maxon EC45 flat, P/N:397172), to the motor mount 403, and a shaft coupler inside the motormount 403 connects the motor 402 shaft to the end of the ball screw 404.The motor mount 403 holds a bearing assembly that supports the ballscrew's axial load. The ball nut travels along the ball screw 404 as theball screw 404 rotates, and the ball nut connects to the input block 406through a pin joint. In an embodiment, the ball screw 404 has a lead of2 mm. Four black carbon fiber plates bolted to the input block 406 clampthe inner ends of the DESs 410 in place. The outer ends of the DESs 410are similarly clamped to the output block 408, and the two portions ofthe output block 408 are connected with long bolts and nuts. Thisconnection makes it possible to adjust the prestrain of the DESs 410.The load attaches to the DES VSA 400 at the output connection point 409,which is a pin connection in an embodiment.

Deformation of a DES 410 changes its capacitance. This change can bemeasured and used to calculate the force output of the DES VSA 400. Thisprocess is similar to how the force output of a series elastic actuator(SEA) is obtained from Hooke's law and measurement of the SEA spring'sdisplacement. However, the calculation for the DES VSA 400 is morecomplex because of the viscoelastic nature of the DESs 410.

Turning now to FIG. 5, a perspective view of a DES VSA 500 is shown inaccordance with an illustrative embodiment. DES VSA 500 may beimplemented as SEA 200 depicted in FIG. 2. DES VSA 500 includes a motor502, a motor mount 503, a ball screw 504, an input block 506, an outputblock 508, an output connection point 509, a plurality of DES diaphragmmodules 510 that each include a DESsheet, and guide rods 511. The DESdiaphragm modules 510 form a variable stiffness mechanism. In contrastto the configuration of DES VSAs 300 and 400 in which the DESs 310, 410were oriented such that the primary plane of each sheet of DES 310, 410was parallel to the direction of force applied by the motor 302, 402,the DESsheets on the DES diaphragm modules 510 are arranged in a stacksuch that the primary plane of each DESsheet of the DES diaphragmmodules 510 is orthogonal to the direction of force applied by the motor502.

FIG. 6A is a diagram of a DES module 600 depicted in accordance with anillustrative embodiment. DES module 600 may be implemented as any of DESdiaphragm modules 510 depicted in FIG. 5. DES module 600 includes anouter frame 602, an inner frame member 606, and DES 604. Althoughdepicted as a substantially circular disk or cylindrical disk, the DES604 and the inner frame member 606 may be fabricated into other shapesin other embodiments. The outer frame 602 is depicted as a hexagonalshape, but it may take the form of other shapes in other embodiments. Inan embodiment, the DES 604 includes a dielectric elastomer layer, afirst compliant electrode layer covering a first surface of theelastomer layer, and a second compliant electrode layer covering asecond surface of the elastomer layer. The first and second compliantelectrode layers are configured to provide a voltage difference across aplane of the dielectric elastomer layer, wherein an increasing voltagedifference causes an increase in softening of the DES.

FIG. 6B is a diagram of an alternate DES module 650 depicted inaccordance with an illustrative embodiment. DES module 650 may beimplemented as any of DES diaphragm modules 510 depicted in FIG. 5. DESmodule 650 includes an inner frame member 656 and an outer frame 652connected to the inner frame member 656 by a DES membrane 655 havingsegmented electrodes 654. By including segmented electrodes, thisconfiguration adds additional actuation degrees of freedom. The innerframe member 656 can be translated within the plane of the module byselectively charging only a portion of the electrodes. DES module 650 issimilar to DES module 600 except that the electrodes 654 are segmentedrather than being a single electrode covering the entire active area.

The design of the disclosed DES VSA 500 and DES module 600 allowsindependent control of stiffness and equilibrium position, unlike anyother DES VSA. Thus, in an embodiment, the DES VSA 500 is configuredsuch that an applied stiffness of the DES diaphragm module 510 isindependent of a force applied to the DES diaphragm module 510. The DESVSA 500 is designed to be a linear actuator to fit the linear motion ofDES diaphragm modules. To create linear motion, a direct drive ballscrew 504 converts the rotation and torque of the actuation motor 502(in an embodiment, the motor 502 is implemented as a Maxon EC45 Flat, 70W) into displacement and force applied to the variable stiffnessmechanism. In this arrangement, the actuation motor 502 sets theequilibrium position of the DES VSA 500 and supplies the force tomaintain that position. The variable stiffness mechanism controls theactuator's stiffness and transmits the force to the load. Alinear-bearing guide-rod 511 system serves as the backbone of theactuator maintaining its components in alignment. In an embodiment, thelinear-bearing guide-rod system 511 is configured to constrain an outputpoint of the DES VSA 500 to linear motion. In an embodiment, the DES VSA500 is mounted with two pin joints, one on its motor mount 503, and theother at the output point 509. In an embodiment, the DES VSA 500 has amass of 880 g and is 45 cm long at maximum extension. In an embodiment,the DES VSA's 500 output point 509 can travel 90 millimeters (mm), andthe DES VSA's 500 equilibrium position travel is 42 mm. In anembodiment, the variable stiffness mechanism can deflect up to 25 mm ineither direction within the DES VSA's 500 range of travel. In anembodiment, the DES VSAs 500 width is 134 mm, and its height is 108 mm.

Returning to FIG. 6A, The DES diaphragm modules 600, which form thedisclosed DES VSA's 500 variable stiffness mechanism in FIG. 5, aresimple mechanisms. In an embodiment, in each module 600, a thin,pre-stretched, adhesive, dielectric elastomer layer 604 (elastomer layer604 may be implemented as, for example, VHB 4910) connects the innerframe member 606 to the outer frame 602. The elastomer layer 604 iscoated on its top and bottom faces with conductive graphite powder,which forms the electrodes of the DES 600. In an embodiment, theelectrodes of the DES 600 are compliant electrode layers. Polyimide filmlines the edges of the elastomer layer 604, reinforcing them against theelectrical field and mechanical stress concentrations that occur there.During operation, the inner frame member 606 displace out of plane, likethe motion of the center of a speaker cone, stretching the DES 604. Whena high-voltage (in an embodiment, over 1000 Volts) charge is applied toits electrodes, the DES 604 softens and relaxes its pretension softeningthe module. Removing the charge stiffens the module 600. Electrically, amodule 600 acts like a capacitor, and its capacitance increases when itsinner frame member 606 is displaced out of plane.

Turning now to FIG. 7, a cross-section view of a DES VSA 700 is depictedin accordance with an illustrative embodiment. DES VSA 700 may beimplemented as, for example, DES VSA 500 depicted in FIG. 5. DES VSA 700is an example of a DES VSA such as DES VSA 200 depicted in FIG. 2.

DES VSA 700 includes an actuation motor 702, a pin joint 703, a ballscrew 704, a linear-bearing guide rod system 706, a variable stiffnessmechanism 708, and an output point 710. The variable stiffness mechanism708 includes one or more DES modules described in more detail below withreference to FIGS. 8A and 8B. The DES VSA 700 also includes a stiffnesscontroller (not shown). The stiffness controller includes a power supplyand is configured to apply a voltage across a variable stiffnessdielectric elastomer membrane. The amount of voltage controls determinesthe amount of softening (i.e., increasing flexibility) of the DES. Morevoltage results in more softening. No applied voltage results in theDESstiffening. Thus, the stiffness of the DES is controlled by varying acontrol voltage applied to the DES. In an embodiment, the stiffnesscontroller is configured to control the stiffness of the DESsuch thatthe stiffness of the DES is at a maximum stiffness when the DES isunpowered. In an embodiment, the stiffness controller controls thestiffness of the DES without the aid of the actuation motor 702 andwithout the aid of any other motor. In an embodiment, the DES isconfigured to measure the force output of the actuation motor with anadditional position sensor, but without any force sensors.

The actuation motor 702 is mechanically coupled to the ball screw 704and is configured to cause a force to be applied to an inner framemember of the flexible membrane via the ball screw 704. An example of aninner frame member is center disk 806 depicted in FIG. 8 and describedbelow. The actuation motor 702 is also configured to control anequilibrium length of the DES VSA 700.

In some embodiments, the DES VSA 700 is connected to a data processingsystem, such as, for example, data processing system 1200 shown in FIG.12 and described below. The data processing system may control actuationmotor 702 and the stiffness controller, thereby controlling theoperation of the DES VSA 700.

FIG. 8 is a cross-sectional view of a DES module 800 that is depicted inaccordance with an illustrative embodiment. DES module 800 may beimplemented as DES module 600 depicted in FIG. 6. DES module 800 may beincorporated into the variable stiffness mechanism 708 of the DES VSA700 depicted in FIG. 7. The DES module 800 includes an outer frame 802,a DES flexible sheet 804, and a center disk 806. The center disk 806 andthe outer frame 802 may be constructed from a plastic such as ABSplastic. Other stiff materials can be used if they are not electricallyconductive. In other embodiments, the center disk 806 and the outerframe 802 may be constructed from an electrically conductive material ifadditional insulating components are used to electrically insulate thecenter disk 806 and the outer frame 802 from the flexible sheet 804. Insome embodiments, the center disk 806 is not placed in the center of theflexible sheet 804. In some embodiments, the center disk 806 is replacedby another inner frame member that has a shape other than circular ordisk like.

The DES flexible sheet 804 includes a dielectric elastomer 808. The DESflexible sheet 804 also includes compliant electrodes 810 surrounding atleast a portion of either side of the DE sheet 808. Additionally, theDES flexible sheet 804 includes a polyimide sheet 812 disposed between aportion of the DE sheet 808 and one of the compliant electrodes 810. Thepolyimide provides mechanical and electrical protection to the DE sheet808. In an embodiment, the DE sheet 808 may include silicone or anacrylic elastomer. In an embodiment, the DE sheet 808 may includefluorosilicones, polyurethanes, or natural rubbers. In some embodiments,silicone may be preferred since silicones typically have much lessviscosity than the acrylic elastomers. In an embodiment, the acrylicelastomer is Very High Bond (VHB) 4905 or VHB 4910, both available from3M. For use in a DES VSA, in an embodiment, an elastomer should have alarge strain capacity, a high relative permittivity, and a highbreakdown field. Additionally, in an embodiment, low viscosity is oftendesirable.

The center disk 806 may displace out of plane as shown in FIG. 8. In anembodiment, the thickness of the outer frame 802 is around 1.9 mm, adiameter of the center disk 806 is about 25.4 mm, and a diameter of theouter frame is about 82.6 mm. In an embodiment, the polyimide sheet 812extends away from the outer frame 802 towards the center disk 806 byabout 2.54 mm. In an embodiment, the polyimide sheet 812 also extendsaway from the center disk 806 toward the outer frame 802 by about 2.54mm. The polyimide sheet 812 does not extend all the way from the outerframe 802 to the center disk 806.

In an embodiment, the variable stiffness mechanism 708 includes of astack of thirty DES modules 800 capped on the ends by two insulatingmodules. The DES modules 800 are variable stiffness modules. In otherembodiments, the number of DES modules 800 may be more or less thanthirty. In an embodiment, the stack of DES modules 800 is arranged in astack structure such that they form a system of mechanically parallelsprings. In other embodiments, the variable stiffness mechanism 708includes a number of DES modules 800, where a number is one or more. Thevariable stiffness mechanism 708 has no rolling or sliding components,so it is mechanically simple. Electrically, the DES modules 800 areconnected in parallel, so they all charge and discharge together. Themodules' 800 center disks 806 are connected to the DES VSA's ball screw704, and the module frames 802 are connected to the actuator's outputpoint 710 as shown in FIG. 7. Thus, the DES modules 800 add their forcetogether when stretched, so they are mechanically parallel springs.

The coupling between a DES VSA's 700 stiffness and its equilibriumposition can be evaluated by fixing the DES VSA's 700 equilibriumposition and then perturbing the DES VSA's 700 output point with a rangeof stiffness settings and repeating for additional equilibriumpositions.

FIG. 9 is a plot of a VSA signature showing independent modulation ofstiffness and equilibrium position, according to test results obtainedfrom an illustrative embodiment of the DES VSA. This plot 900 of theload applied to the DES VSA 500 and its output point displacement can beregarded as its signature.

The plot 900 shown in FIG. 9 of the reaction force and output pointdisplacement resulting from these perturbations of an embodiment DES VSA700 shows the range of stiffnesses the DES VSA 700 can reach at eachequilibrium setting and can be regarded as the DES VSA's signature. Thestiffnesses of a DES VSA appear in its signature as the slope of thenon-zero force portions of the force-displacement trajectories, and theequilibrium points appear as displacement values where force is zero.

One signature of DES VSA 500 has a pair of stiffer and softertrajectories that originate from equilibrium (Equi.) point 1 at 0 mm.The stiffer curve was generated with DES VSA's 500 variable stiffnessmechanism discharged, and the softer curve with the mechanism charged to5.0 kV. This feature of the signature shows that the DES VSA 500 canmodulate its stiffness. The mechanism can also have intermediatestiffness values, but we omitted them here so as not to clutter theplot. The VSA signature of the disclosed DES VSA 500 has a range ofzero-force points between equilibrium points 1 and 2 that was generatedby shifting the DES VSA's 500 equilibrium point. This feature shows thatthe DES VSA 500 can control its equilibrium point, making it anactuator. Finally, the signature has another pair of stiff and softforce-displacement trajectories that originate from equilibrium point 2.These curves are identical to those originating from equilibrium point1, so the signature shows that the DES VSA 500 can reach its full rangeof stiffnesses across its range of equilibrium positions. Therefore, ourDES VSA 500 can modulate its stiffness and equilibrium positionindependently.

FIGS. 10A, 10B, and 10C show plots 1002, 1004, 1006 of tensile testresults showing the stiffness change of an embodiment of the DES VSA.When the variable stiffness mechanism is charged with a constantvoltage, it softens as can be seen from these test results. In thesetests, the equilibrium position of the DES VSA is constant, and thevariable stiffness mechanism is displaced at a constant speed of (A) 1mm/s, (B) 10 mm/s, or (C) 100 mm/s. The displacement of the variablestiffness mechanism is on the horizontal axis, while the load applied ison the vertical axis. Both are linearly scaled. The peak forces aremarked on the vertical axis. Notice also that the variable stiffnessmechanism exerted more force during high speed motion due to theviscoelasticity of the dielectric elastomer material in the DES modules.

The variable stiffness mechanism softens up to 52% shown in Table IIbelow when charged to 5.0 kV, according to a series of tensile tests1002, 1004, 1006 shown in FIGS. 10A, 10B, and 10C. Itsforce-displacement relationship for a constant applied voltage isnonlinear, so its stiffness is a function of its displacement. Themechanism's stiffness reduces most at small deflections, 52% for the 0-5mm interval. At larger deflections, the stiffness reduction diminishes,and stiffness increases by 2.5% for the 20-25 mm interval. Noise in thedata makes numerical differentiation of the force-displacement curvesusing neighboring data points inaccurate, so we calculated an averagestiffness for 5 mm intervals from the force and displacement values atthe endpoints of those intervals. The data in Table II was calculatedfrom the portion of the lmm/s trajectories that correspond to extension(moving away from equilibrium) and tension (positive force in FIGS. 10A,10B, and 10C). We chose this portion of the data because it is the leastinfluenced by the viscosity of the modules and friction from thetestbed. Only data for 0 kV and 5 kV is reported in FIGS. 10A, 10B, and10C so as not to clutter the plots 1002, 1004, 1006. However,intermediate stiffness reductions were also obtained when the variablestiffness mechanism was charged to 2 kV and 4 kV as seen in plot 1100 inFIG. 11.

TABLE II Stiffness Change for During Tensile Test at 1 mm/s, and %Change Relative to 0-kV Stiffness Displacement Average Stiffness (N/mm)% change of stiffness Interval (mm) 0 kV 2 kV 4 kV 5 kV 2 kV 4 kV 5 kV0-5 4.42 3.77 2.86 2.10 −14.5 −35.1 −52.4  5-10 4.93 4.62 3.56 2.72 −6.3−27.8 −44.8 1.0-15  5.26 5.25 4.68 4.35 −0.2 −11.0 −17.4 15-20 6.59 6.235.96 6.05 −5.5 −9.5 −8.1 20-25 7.89 7.86 7.99 8.09 −0.4 1.3 2.5

Repeating the tensile tests at higher speeds showed the effects ofviscoelasticity in the variable stiffness mechanism. FIG. 10B and FIG.10C show that the force output and hysteresis of the variable stiffnessmechanism increase when it is displaced more rapidly. Theviscoelasticity of the dielectric elastomer used in the DES modulesexplains these two effects because viscosity damps motion.

According to the tensile test data, an embodiment of the disclosed DESVSA could supply a steady force of 140N without exceeding thedisplacement limits of its variable stiffness mechanism. Due to theviscoelasticity of the DES modules, an embodiment of the disclosed DESVSA can briefly sustain loads greater than 140N, but such loads willoverextend the variable stiffness mechanism and may damage it if appliedtoo long. The value of 140N is the force value at the start ofretraction (moving towards equilibrium) during the tension half-cyclefor the lmm/s tensile test (FIG. 10A). Viscous damping caused force toreach 150N during extension of the variable stiffness mechanism, butthis force dropped to 140N while the mechanism remained at maximumdisplacement. Compression makes the DES VSA and load actuator systemtend to buckle, so it is believed that the additional force magnitudeduring compression is caused by friction from the supports that resistbuckling. The DES modules are essentially symmetric with respect to theresting plane of their diaphragms (FIG. 8), so they should exert thesame force magnitude for either displacement direction and should notcause the additional compression force.

Integration of the force-displacement curves in FIGS. 10A, 10B, and 10Cyields the mechanical energy absorbed or returned by the variablestiffness mechanism shown in Table III below. For the lmm/s trial atOkV, the variable stiffness mechanism absorbed 1.67 J, storing a portionelastically and viscously dissipating the rest, and returned 1.46 J ofmechanical energy. Since this trial was quasistatic, minimizing viscouslosses, these values approximate the lower bound for energy absorbed andupper bound for energy returned when the variable stiffness mechanism isat its stiffest setting. When the variable stiffness mechanism wasdisplaced more rapidly, it absorbed more energy while stretching andvoltage the variable stiffness mechanism was charged to, and thedisplacement applied to it. Both energy and power increase as voltage ordisplacement increases. At most, the mechanism returned less energywhile returning to equilibrium as seen in the data from the 10 mm/s and100 mm/s trials at 0 kV.

TABLE III The Electrical Energy Required to Reduce Stiffness and thePower Required to Hold the Reduced Stiffness VSM Disp. Energy (J) Power(mW) (mm) 2.0 4.0 5.0 2.0 4.0 5.0 5 0.182 0.734 1.159 39 164 262 kV, VSM10 0.222 0.799 1.230 40 161 267 voltage 20 0.327 1.253 1.764 46 173 30525 0.468 1.494 2.022 54 173 324

FIG. 11 shows a plot 1100 of tensile test results for multiple voltages.These plots show that intermediate voltages between 0 kV and 5 kVproduce intermediate stiffnesses. As for the results in FIG. 10A, thevariable stiffness mechanism was displaced at 1 mm/s from −25 mm to 25mm. However, only the tension portion is shown here so that the resultscan be seen clearly.

The mechanical simplicity that the disclosed embodiments provide is anattractive design feature. Because the motion in the DES modules occursthrough stretching instead of rolling or sliding, the DES variablestiffness mechanism does not need bearings or bushings, which can wearout. Further, the mechanism does not need complex and costly machining.In an embodiment, assembly of the DES modules includes merely 3Dprinting, laser cutting, and hand assembly.

Despite the mechanical simplicity of its variable stiffness mechanism,the functionality of the disclosed DES VSA matches that ofstate-of-the-art VSAs. In an embodiment, the VSA signature of thedisclosed DES VSAs displayed independent control of stiffness andequilibrium position, and the DES VSA sustained 140N of steady load, afeature combination that no prior DES VSA can match. In an embodiment,the energy absorption (≥1.67 J) and return (≤1.46 J) capacity of thedisclosed DES VSA's variable stiffness mechanism falls within the rangeachieved by state-of-the-art VSAs, 0.19 J to 8.5 J for example. Anembodiment of the disclosed DES VSA achieved a 52% stiffness reductionfor extensions up to 5 mm, which is less than the “infinite” stiffnessvariation some VSA's are capable of. However, other work has shown thatDESs can achieve zero stiffness for certain ranges of motion. Due tolack of published data, it is difficult to compare the power thedisclosed DES VSA requires to hold reduced stiffness to the state of theart. However, the range of power measured (324-39 mW) is two to threeorders of magnitude less than the VSA's actuation motor rating (70W),which implies that stiffness holding may not greatly affect theactuator's overall power consumption. Because a single DES modulesoftened in 27 ms, it is inferred that the DES VSA could soften in aslittle as 27 ms with a sufficiently powerful high voltage power supply.This performance would be on par with that of antagonistic VSAs andexceed that typical of VSAs with dedicated stiffness modulation motors.

The stiffness changing behavior of the disclosed DES VSAs is unlike thatof any state-of-the-art VSA. When stiffening under load,state-of-the-art VSAs do positive work on their springs, so they requireenergy to stiffen. When softening under load, they do negative work ontheir springs and may recover energy from them. Therefore, they are“default-soft” VSAs. In contrast, the disclosed DES VSA is“default-stiff” because it requires energy to soften and may recoverelectrical energy from its DES modules when it stiffens. Default-stiffbehavior could be an advantage for certain applications. Roboticprostheses and orthoses for legs should default to stiff settings whenthey lose power to maintain support for their wearer. They may also needto maintain stiff settings for long periods when their wearer isstanding still, which could be energetically costly for a default-softVSA, since doing so implies stalling a motor (or activating a clutch,which most VSAs do not have), but could require little or no stiffnessmodulation power for a default stiff VSA. Broadly, an application thatrequires a VSA to be stiff much more often than soft would tend tobenefit from a default-stiff VSA.

In one illustrative example, one or more technical solutions are presentthat overcome technical problems with the weight, complexity, anddefault flexible when unenergized of prior art VSAs. As a result, one ormore technical solutions may provide a technical effect of a DES VSAthat defaults to default stiff when unenergized. The disclosed DES VSAsare also mechanically simpler than prior art VSAs.

The illustration of the DES VSAs and DES modules in the FIGS. 2-9 is notmeant to imply physical or architectural limitations to the manner inwhich an illustrative embodiment may be implemented. Other components inaddition to or in place of the ones illustrated may be used. Somecomponents may be unnecessary. Also, the blocks are presented toillustrate some functional components. One or more of these blocks maybe combined, divided, or combined and divided into different blocks whenimplemented in an illustrative embodiment.

Turning now to FIG. 12, a block diagram of a data processing system isdepicted in accordance with an illustrative embodiment. Data processingsystem 1200 may be implemented to control the motors or stiffness of theDESs in the DES VSA 200 depicted in FIG. 2, DES VSA 300 depicted in FIG.3, DES VSA 400 depicted in FIG. 4, DES VSA 500 depicted in FIG. 5, orDES VSA 700 in FIG. 7. Data processing system 1200 may be implemented asan electric motor controller and/or as a stiffness controller. In thisillustrative example, data processing system 1200 includescommunications framework 1202, which provides communications betweenprocessor unit 1204, memory 1206, persistent storage 1208,communications unit 1210, input/output (I/O) unit 1212, and display1214. In this example, communications framework 1202 may take the formof a bus system.

Processor unit 1204 serves to execute instructions for software that maybe loaded into memory 1206. Processor unit 1204 may be a number ofprocessors, a multi-processor core, or some other type of processor,depending on the particular implementation.

Memory 1206 and persistent storage 1208 are examples of storage devices1216. A storage device is any piece of hardware that is capable ofstoring information, such as, for example, without limitation, at leastone of data, program code in functional form, or other suitableinformation either on a temporary basis, a permanent basis, or both on atemporary basis and a permanent basis. Storage devices 1216 may also bereferred to as computer-readable storage devices in these illustrativeexamples. Memory 1206, in these examples, may be, for example, arandom-access memory or any other suitable volatile or non-volatilestorage device. Persistent storage 1208 may take various forms,depending on the particular implementation.

For example, persistent storage 1208 may contain one or more componentsor devices. For example, persistent storage 1208 may be a hard drive, asolid-state drive (SSD), a flash memory, a rewritable optical disk, arewritable magnetic tape, or some combination of the above. The mediaused by persistent storage 1208 also may be removable. For example, aremovable hard drive may be used for persistent storage 1208.

Communications unit 1210, in these illustrative examples, provides forcommunications with other data processing systems or devices. In theseillustrative examples, communications unit 1210 is a network interfacecard.

Input/output unit 1212 allows for input and output of data with otherdevices that may be connected to data processing system 1200. Forexample, input/output unit 1212 may provide a connection for user inputthrough at least one of a keyboard, a mouse, or some other suitableinput device. Further, input/output unit 1212 may send output to aprinter. Display 1214 provides a mechanism to display information to auser.

Instructions for at least one of the operating system, applications, orprograms may be located in storage devices 1216, which are incommunication with processor unit 1204 through communications framework1202. The processes of the different embodiments may be performed byprocessor unit 1204 using computer-implemented instructions, which maybe located in a memory, such as memory 1206.

These instructions are referred to as program code, computer usableprogram code, or computer-readable program code that may be read andexecuted by a processor in processor unit 1204. The program code in thedifferent embodiments may be embodied on different physical orcomputer-readable storage media, such as memory 1206 or persistentstorage 1208.

Program code 1218 is located in a functional form on computer-readablemedia 1220 that is selectively removable and may be loaded onto ortransferred to data processing system 1200 for execution by processorunit 1204. Program code 1218 and computer-readable media 1220 formcomputer program product 1222 in these illustrative examples. In theillustrative example, computer-readable media 1220 is computer-readablestorage media 1224.

In these illustrative examples, computer-readable storage media 1224 isa physical or tangible storage device used to store program code 1218rather than a medium that propagates or transmits program code 1218.

Alternatively, program code 1218 may be transferred to data processingsystem 1200 using a computer-readable signal media. Thecomputer-readable signal media may be, for example, a propagated datasignal containing program code 1218. For example, the computer-readablesignal media may be at least one of an electromagnetic signal, anoptical signal, or any other suitable type of signal. These signals maybe transmitted over at least one of communications links, such aswireless communications links, optical fiber cable, coaxial cable, awire, or any other suitable type of communications link.

The different components illustrated for data processing system 1200 arenot meant to provide architectural limitations to the manner in whichdifferent embodiments may be implemented. The different illustrativeembodiments may be implemented in a data processing system includingcomponents in addition to or in place of those illustrated for dataprocessing system 1200. Other components shown in FIG. 12 can be variedfrom the illustrative examples shown. The different embodiments may beimplemented using any hardware device or system capable of runningprogram code 1218.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiment. The terminology used herein was chosen to best explain theprinciples of the embodiment, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed here.

What is claimed is:
 1. A variable stiffness actuator (VSA) comprising:an input block and an output block; a plurality of stacked dielectricelastomer system (DES) diaphragm modules positioned between the inputblock and the output block, each DES diaphragm module including an outerframe, an inner frame, and a variable stiffness membrane spanningbetween the outer frame and the inner frame, the variable stiffnessmembrane configured to soften when energized and stiffen when notenergized, wherein one of the input block or output block is coupled tothe outer frames and another of the input block or output block iscoupled to the inner frames; and an actuator mechanically coupled to theinput block, the actuator configured to move the input block in adirection substantially parallel with a stacked direction of the DESdiaphragm modules.
 2. The VSA of claim 1, further including a stiffnesscontroller coupled to the plurality of stacked DES diaphragm modules,the stiffness controller configured to adjust the stiffness of thevariable stiffness membranes by varying a control voltage appliedthereto.
 3. The VSA of claim 1, wherein the stiffness of the variablestiffness membranes is at a maximum stiffness level when not energized.4. The VSA of claim 1, wherein the stiffness of the variable stiffnessmembranes is controlled without aid of a motor.
 5. The VSA of claim 1,wherein a force output of the actuator is measured without separateforce sensors.
 6. The VSA of claim 1, wherein the actuator is anactuation motor, and further wherein the input block is coupled to theinner frames and the output block is coupled to the outer frames, andfurther including a ball screw coupled between the actuation motor andthe input block, wherein the actuation motor is configured to drive theball screw thereby applying a force to the inner frames.
 7. The VSA ofclaim 6, further including an output point connected to the outputblock, the output point configured to connect the VSA to a load.
 8. TheVSA of claim 1, further including a linear-bearing guide-rod systemconnecting to a mount of the actuator and configured to constrain anoutput point of the VSA to linear motion.
 9. The VSA of claim 1, furtherincluding a stiffness controller coupled to the plurality of stacked DESdiaphragm modules, the stiffness controller configured to adjust thestiffness of the variable stiffness membranes by varying a controlvoltage applied thereto, wherein the actuation motor, the plurality ofstacked DES diaphragm modules, and the stiffness controller areconfigured such that the equilibrium position of the plurality ofstacked DES diaphragm modules and the stiffness of the plurality ofstacked DES diaphragm modules are independently controllable.
 10. TheVSA of claim 1, wherein an applied stiffness of the plurality of stackedDES diaphragm modules is independent of an applied force to theplurality of stacked DES diaphragm modules.
 11. The VSA of claim 1,wherein the variable stiffness membranes each comprise an elastomerlayer, a first compliant electrode layer covering a first surface of theelastomer layer, and a second compliant electrode layer covering asecond surface of the elastomer layer, the first and second compliantelectrode layers configured to provide a voltage difference across aplane of the elastomer layer, wherein an increasing voltage differencecauses an increase in softening of the elastomer layer.
 12. The VSA ofclaim 1, wherein the plurality of stacked DES diaphragm modules areelectrically coupled to a stiffness controller in parallel such that theplurality of stacked DES diaphragm modules charge and dischargetogether.
 13. The VSA of claim 1, wherein the variable stiffnessmembranes comprise one of an acrylic elastomer, silicone, afluorosilicone, a polyurethane, and a natural rubber.
 14. The VSA ofclaim 1, wherein the variable stiffness membranes comprise Very HighBond (VHB)
 4910. 15. The VSA of claim 1, wherein the inner framescomprise a substantially circular disk.
 16. The VSA of claim 1, whereinthe inner frames are situated substantially in a center of the outerframes.
 17. The VSA of claim 1, further including a polyimide filmlining edges of the plurality of stacked DES diaphragm modules, therebyreinforcing the plurality of stacked DES diaphragm modules against anelectrical field and mechanical stress concentrations that occur nearthe edges of the plurality of stacked DES diaphragm modules.
 18. The VSAof claim 1, wherein the actuator is an actuation motor.
 19. The VSA ofclaim 1, wherein the input block is coupled to the inner frames and theoutput block is coupled to the outer frames.
 20. A variable stiffnessactuator (VSA) comprising: a plurality of stacked dielectric elastomersystem (DES) diaphragm modules, each DES diaphragm module including avariable stiffness membrane comprising a material that will soften whenenergized and stiffen when not energized; a stiffness controllerconnected to the plurality of stacked DES diaphragm modules andconfigured to control the stiffness of the DES diaphragm modules; anactuator mechanically coupled to the plurality of stacked DES diaphragmmodules and configured to control an equilibrium position of the VSA,the actuator configured to move in a direction substantially parallelwith a stacked direction of the DES diaphragm modules; and a connectorconnecting the plurality of stacked DES diaphragm modules to a load. 21.The VSA of claim 20, wherein the actuator is an actuation motor.
 22. Avariable stiffness actuator (VSA) comprising: a plurality of stackeddielectric elastomer system (DES) diaphragm modules, each DES diaphragmmodule including a variable stiffness membrane, wherein a first side ofeach of the variable stiffness membranes is at least partially coveredby a first compliant electrode, wherein a second side of each of thevariable stiffness membranes is at least partially covered by a secondcompliant electrode, and wherein each of the variable stiffnessmembranes is configured to soften when energized with an electric fieldapplied via the first and second compliant electrodes and to becomestiffer when unenergized; a stiffness controller connected to theplurality of stacked DES diaphragm modules, the stiffness controllerconfigured to control the stiffness of the variable stiffness membranesvia application of a control voltage thereto; a ball screw; an actuatorcoupled to the ball screw, the actuator configured to adjust anequilibrium position of the VSA by moving in a direction substantiallyparallel with a stacked direction of the DES diaphragm modules; an inputblock configured to mechanically couple the ball screw to one of innerframes or outer frames of the plurality of stacked DES diaphragmmodules; an output block configured to mechanically couple the other ofthe outer frames or the inner frames of the plurality of stacked DESdiaphragm modules to an output connection point, the output connectionpoint configured to receive a load.
 23. The VSA of claim 22, wherein theactuator is an actuation motor.
 24. The VSA of claim 22, wherein theinput block is coupled to the inner frames and the output block iscoupled to the outer frames.